Voltage controlled oscillators (VCO) generally have a non-linear tuning characteristic, which means that the transfer function between the tune voltage input and frequency output is non-linear. This tuning characteristic is commonly linearized and temperature compensated using PROMs (Programmable Read Only Memories).
An ESM receiver is often a scanning superheterodyne type receiver which is used to intercept and derive information from RF and microwave signals.
Most ESM Receivers require one or more VCOs to provide the function of the local oscillator. These systems require the voltage input/frequency output transfer function to be linearized and temperature compensated. This function has been traditionally implemented with PROMs or breakpoint generators. This is expensive in terms of hardware cost, testing time, and does not work if some component characteristic drifts with age. In addition, if a certain components within the VCO fail, the calibration data in the PROM becomes invalid. New linearization and temperature compensation data must be retaken and input to the PROM.
The use of programmed control of a VCO with a varactor tuned filter has been illustrated in a number of U.S. Patents. U.S. Pat. No. 4,641,101, issued Feb. 3, 1987, to Harold N. Selim provided a VCO with a programmable tuner which generated voltages for two voltage dividers. A tuning curve was programmed in a RAM (Random Access Memory) with tuning voltage data values stored in memory to cause the dividers to follow the input. There the VCO tuning voltage was quantitized into sixteen steps.
This may be contrasted with the approach of shaping networks and PROMs incorporated into microprocessor controlled signal generators mentioned, along with thermistors for temperature correction, in U.S. Pat. No. 4,728,906, issued Mar. 1, 1988 to Turl et al. That patent controlled a varactor-tuned oscillator by using look-up tables for fine tuning with a program for a microprocessor to perform the various frequency establishing steps and calibration steps of the apparatus.
Microcomputers have been used to measure the frequency of an output from a VCO, as illustrated by U.S. Pat. No. 4,823,399 issued Apr. 18, 1989 to Ashok K. George.
Programmed tuning has been used in tuning television oscillators, as illustrated by the U.S. Patents to Rast et al., U.S. Pat. Nos. 4,078,212 issued Mar. 7, 1978 and 4,997,008.
The aforesaid programatical and hardware approaches are an expensive solution to linearizing a VCO throughout its frequency range and requires additional elements to handle temperature compensation.